The present invention relates generally to a method for the regeneration of an abrasive slurry utilized in the preparation of silicon wafers. More particularly, the present invention relates to a method of separating components of an exhausted slurry, used in slicing wafers from a single crystal or polycrystalline silicon ingot, such that the desirable abrasive grains and lubricating, or cooling, fluid therein may be reused.
Silicon wafers are obtained from a single crystal or polycrystalline silicon ingot by first slicing the ingot in a direction normal to its axis. Typically, the slicing operation is accomplished by means of a wire saw, wherein the silicon ingot is contacted with a reciprocating wire while a slurry containing abrasive grains is supplied to the contact area between the ingot and the wire. Conventional wire saw slurries typically comprise a lubricating, or cooling fluid, such as, for example, mineral oil or some water soluble liquid (e.g., polyethylene glycol, or “PEG”), and abrasive grains, such as silicon carbide.
During the slicing operation, silicon particulate from the ingot and metal particulate (primarily iron) from the slicing wire and metal pipes form and are incorporated into the slurry. As the concentrations of the silicon and metal particulate in the slurry increase, the efficiency of the slicing operation decreases. Eventually, the slurry becomes ineffective, or “exhausted,” and must be discarded. Typically, the exhausted slurry is disposed of by incineration or treated by a waste water treatment facility. However, burning this slurry generates carbon dioxide and sending this slurry to a waste water treatment facility typically results in the formation of a sludge which must be disposed of in a landfill. Accordingly, both approaches of disposal are unfavorable from an environmental point of view. It is therefore desirable to eliminate, or significantly reduce the amount of, this waste which is generated.
In addition to the negative environmental consequences and the costs associated with the disposal of the exhausted slurry, silicon wafer production costs are also increased due to the fact that the slurry is generally disposed of prematurely. More specifically, the rate at which the abrasive grains in the slurry are “spent” (i.e., worn to a size too small for effective slicing) by the slicing operation is generally much less than the rate at which the slurry becomes contaminated by silicon and metal particulate. Furthermore, the useful life of a typical lubricating or cooling fluid is dictated almost entirely by the build-up of silicon and/or metal particulate; that is, the lubricating or cooling solution could be used for a much longer period of time, if it were not for the increasing concentration of silicon and metal particulate in the slurry. As a result, the slurry is typically discarded once the level of silicon and/or metal particulate in the slurry is too high, even though much of the abrasive grains and lubricating fluid are still usable.
In view of the foregoing, a need continues to exist for a method which may be utilized to separate silicon and metal wire particulate from the slurry, thus enabling the reuse of abrasive grains and the lubricating fluid. Such a process would reduce the manufacturing costs associated with the slicing of silicon ingots. In addition, such a process would reduce the amount of waste product and/or waste byproduct emitted into the environment.